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Abstract

In optical coherence tomography (OCT), axial resolution is one of the most critical parameters impacting image quality. It is commonly measured by determining the point spread function (PSF) based on a specular surface reflection. The contrast transfer function (CTF) provides more insights into an imaging system’s resolving characteristics and can be readily generated in a system-independent manner, without consideration for image pixel size. In this study, we developed a test method for determination of CTF based on multi-layer, thin-film phantoms, evaluated using spectral- and time-domain OCT platforms with different axial resolution values. Phantoms representing six spatial frequencies were fabricated and imaged. The fabrication process involved spin coating silicone films with precise thicknesses in the 8-40 μm range. Alternating layers were doped with a specified concentration of scattering particles. Validation of layer optical properties and thicknesses were achieved with spectrophotometry and stylus profilometry, respectively. OCT B-scans were used to calculate CTFs and results were compared with convetional PSF measurements based on specular reflections. Testing of these phantoms indicated that our approach can provide direct access to axial resolution characteristics highly relevant to image quality. Furthermore, tissue phantoms based on our thin-film fabrication approach may have a wide range of additional applications in optical imaging and spectroscopy.

OCT images of the six phantoms, with the average A-scans in blue next to the images. All images are 500 μm wide. Images for phantoms A-D are 150 μm optical depth, while those for phantoms E and F are 300 μm optical depth (wS: wideband SDOCT, nS: narrowband SDOCT, wT: wideband TDOCT, nT: narrowband TDOCT).

CTF curves for the four OCT configurations. Error bars represent the standard deviation of each data point. Horizontal error bars are shown only on the wT curve for clarity; they are the same for all curves. Vertical dashed lines indicate the equivalent spatial frequency to the FWHM of each configuration’s axial PSF. Inset shows axial PSFs for the four OCT configurations (wS: wideband SDOCT, nS: narrowband SDOCT, wT: wideband TDOCT, nT: narrowband TDOCT).